1. Field of the Disclosure
The present disclosure relates to a method and apparatus for positioning blades of a gas turbine engine, for example during manufacturing the blades of a rotor for a gas turbine engine. In particular, but not exclusively, this invention can relate to a method of grinding the tips of the blades.
2. Description of the Related Art
A gas turbine engine typically comprises multiple rotor stages and multiple stator stages. The rotor stages comprise blades that rotate within a casing. The efficiency at which the engine operates can be affected by flow leaking between the rotor blades and the casing during use, which may be referred to as over-tip leakage. This may apply both to compressor rotor stages and to turbine rotor stages, but may be particularly important in compressor stages.
In order to minimize such leakage, an abradable liner may be used to circumferentially surround the blades. This abradable liner may be abraded by the tips of the blades in use in order to try to provide a good seal during engine running.
However, in order for the abradable liner to be effective, all of the tips of all of the compressor blades must be at the same radial position, or at least as close to being at the same radial position as possible. Any difference in radial tip position means that the abradable liner is abraded away by the blade tip at the further radial position, which in turn results in a radial gap between the other blade tips and the abradable liner. This radial gap may allow increased over-tip leakage, and thus decreased engine efficiency.
It is therefore important to ensure that the radial positions of the tips of all of the rotor blades in a rotor stage of a gas turbine engine are as close to being the same as possible. To this end, a grinding process may be used on the blade tips during manufacture. Such a grinding process may involve rotating a rotor stage relative to a grinding surface. The grinding surface may thus be used to try to grind the tips of the blades to the same position.
However, a problem occurs during this grinding process which may be said to be because of the way in which the blades are secured into the rotor disc. In particular, the blades are conventionally secured into one or more slots in the rotor disc. For example, each blade may have a root that is secured into a respective slot in the rotor disc that extends in a generally axial direction. Alternatively, each blade may have a root, and the roots of all of the blades may be secured into a single circumferentially extending slot in a rotor disc.
Regardless of whether the roots of the blades are secured in an axially extending slot or a circumferentially extending slot, there is necessarily a degree of play or movement allowed between the blade root and disc slot in order for the blades to be slotted into place during assembly. However, this play or movement means that the blade can adopt a range of different positions when the rotor stage is rotated depending on, for example, exactly where the blade root and disc slot engage as the rotor speed is increased and the blade is centrifuged radially outwardly. Accordingly, during the tip grinding process, the engagement of one blade root in a slot may be at a different position to another blade root in its slot. This may result in the shape and/or absolute radial extent of the blades being different after the grinding process because, although all of their tips may engage the same grinding surface, their root may be in different positions and/or the blades may be at slightly different angles (i.e. they may not all be exactly radial and/or may have different axial lean components, again depending on the engagement position of the blade root with the slot). When the resulting different shaped and/or sized blades are rotated during running of a gas turbine engine, the differences in their geometry may lead to increased unwanted abrasion of the abradable liner. In turn, this may result in increased over-tip leakage, and thus decreased stage and engine efficiency.